1. Field of the Invention
The present invention generally relates to identification tag structures for attaching to an article of interest, and more specifically to radio frequency (RF) magnetic tag structures having portions (e.g., arms) coated with thin magnetic films and utilizing resonant frequencies of the arms for identification of the article.
2. Cross-reference to Related Applications
The invention disclosed and claimed herein is related to the inventions disclosed and claimed in copending applications Ser. Nos 08/344,808, 08/344,296 and 08/344,196.
3. Description of the Related Art
For personnel identification, retail tagging, tagging used in the road/air-freight package industry, and pallet tagging in manufacturing processes, a tag (also known as an "identification tag" or "marker") coupled to animate (e.g., personnel) or inanimate objects (hereafter referred to as "products" or "articles") is useful for identifying the personnel/product/article in detail. With a sufficient number of bits, the tag can be interrogated to determine what the product is (or who the person wearing the tag is), when it was manufactured, its price, whether the product has been properly passed through an interrogation zone (e.g., a check-out counter or a kiosk) etc. Identifying the product via a tag may hasten a new type of checkout system for the retail industry giving rise to a so-called "no-wait checkout".
Conventional magnetic tags and tag systems have had a number of problems including: 1) having only one bit, 2) requiring a large amount of power so as to read the tag, and 3) being relatively easy to defeat by tampering.
For example, a first conventional tag element includes a single longitudinally resonant element, whereas a second conventional tag element includes a plurality of resonant elements. Each of these tags uses soft magnetostrictive material on a substrate, biased by an adjacent strip of a hard magnetic material. Magnetic fields stimulated by an alternating current (AC) field causes rotation of domains resulting in a change in the strip's permeability. This change in permeability can give rise to a resonant magnetomechanical coupling and results in a dimensionally changed and enhanced magnetic flux change which is detected by a receiving coil. Deactivation of the tag is accomplished by using an additional bias magnet located externally to the tag to change the field of the fixed magnet within the tag, thereby resulting in a change in the strip's resonant frequency so that the resonant frequency falls outside of a preset detectable frequency.
The conventional tags above do not employ cantilevers. Further, these conventional tags employ magnetostrictive materials (e.g., materials in which their dimensions change due to magnetization thereof) and utilize only magnetic excitation. Each of the conventional tags above utilizes resonant frequencies, which, because they result in longitudinal rather than transverse resonances, are proportional to the reciprocal length of the magnetostrictive strip.
A third conventional tag has a vibration sensor having one or more cantilevers tuned to resonate at predetermined frequencies. The cantilever vibration causes a circuit gap to close, thereby allowing current to flow through the cantilever and the then-closed contact, with the current then flowing to a microchip and an integrated circuit mounted on the base of the device.
This tag is disadvantageous because it requires a power source, wiring, current flow through the device for sensing and an integrated circuit, all of which are part of the package and thereby make the device large and complex. Excitation is via mechanical vibration.
Yet another conventional tag system includes several different lengths of magnetostrictive material which are made to resonate in a longitudinal mode upon magnetic AC excitation being applied thereto. The resonant frequency is proportional to 1/length as opposed to 1/(length).sup.2 for cantilevers. Because the material is magnetostrictive, a rotation of domains upon application of an external, heterogeneous bias field will cause a change in the Young's modulus of the magnetostrictive material, thereby resulting in a shift in resonance frequencies. As is known, the Young's modulus is dependent on the applied magnetic bias field. The change in Young's modulus in an applied field is commonly referred to as "the .DELTA.E effect". The shift in frequency is sensed upon application of an alternating current (AC) resonant magnetic field in combination with the direct current (DC) field. For an externally applied heterogeneous bias field, more than one tag can be detected even if otherwise identical, so long as each tag is located spatially apart from one another.
A fourth conventional device includes cantilevers having different lengths, responding to different frequencies resulting from speech patterns. The device uses neither magnetic materials nor magnetic excitation or detection. Instead, piezoelectric sensors are incorporated onto the cantilever structure for sensing speech, thus requiring wiring. Further, there is no ability of this device to be sensed remotely.
In view of the foregoing, hitherto the invention, there has not been any tag or tag system having more than one bit, which can be read with relatively low power density (e.g., preferably on the order of .apprxeq.0.1mW/cm.sup.2) and sensed remotely, and which is difficult to defeat by tampering.